A butterfly valve uses a disk which rotates to block flow or allow flow to pass on around the disk. A spherical valve is more closely aligned with a plug or a ball valve. The valve has Stellite and Stainless steel seating surfaces and is used in higher pressure applications. (Greater than 250 psig.)
Further, a butterfly valve is a commodity product. A 42" butterfly valve costs approximately 1/10 that of a spherical valve. Spherical valves are made to order. There are not any good websites but many of the large turbine manufacturers will build you one.
This type of valves has a minimum head loss due to a complete free through flow. They are usually installed as high pressure turbine inlet and pump valves and are equipped with controlled service and maintenance sealings. Flow in both directions is possible. The control system is of oil hydraulic pressure system or by water pressure system or mixed - opening by oil pressure, closing by water pressure. The water for the valve control system is taken from the upstream side. Oil pressure for the control system can be also supplied from the hydraulic turbine governor oil system.
Butterfly valves:-
This type of valves can be installed in a pipelines having fresh water,sea water,corrosive chemicals,alkalies,alcohol & waste water as it's media.It has the most simplest and easy design having a body,a seal,a disc& a stem as it's primary components.The open and closing is always at 90 degrees.The operators can be manual,gear,electrical or hydraulic.the smallest of a butterfly valve can be 50mm to the sumo(just kidding!)category ones which are 5000mm in dia.The beauty of the butterfly valve is that Irrespective of it's size the design can remain the same.
